In 1920, representatives of several professional football leagues and independent teams met in Canton, Ohio, and founded the American Professional Football Conference, soon renamed the National Football League. The first official championship game was held in 1933; before then, there was no playoff system, and instead the team that finished with the best regular season record was awarded the league title. By 1958, after which that season's NFL championship game became known as "The Greatest Game Ever Played", Professional Football was on its way to becoming one of the most popular sports in the United States. A new league, the American Football League, began play in 1960, and its expanded market and offense-oriented style of play had the result that by 1965, Professional Football supplanted Major League Baseball as the most popular televised sport in America.[14] The NFL's merger with the American Football League, agreed to in 1966 and completed in 1970, further expanded the sport and created the Super Bowl, which has become one of the most watched sporting events in the world, and is second to association football (soccer)'s UEFA Champions League final as the most watched annual sporting event worldwide.[15]

Official rules and notable rule distinctions

See also: American football rules and List of NFL nicknames#Rules named after NFL figures

Although rules for NFL, college, and high school football games are generally consistent, there are several differences. In addition, the NFL frequently makes rule changes because of exploits on the field by a single coach, owner, player, or referee.

Some of the major rules differences include:

A pass is ruled complete if both of the receiver's feet are inbounds at the time of the catch. In college and high school football, only one foot is required to be inbounds.

In the NFL, a player is considered down when he is tackled or forced down...